Razor blades are typically formed of a suitable metallic sheet material such as stainless steel, which is slit to a desired width and heat-treated to harden the metal. The hardening operation utilizes a high temperature furnace, where the metal may be exposed to temperatures greater than about 1000° C. for up to about 20 seconds, followed by quenching, whereby the metal is rapidly cooled to obtain certain desired material properties.
After hardening, a cutting edge is formed generally by grinding the blade. The steel razor blades are mechanically sharpened to yield cutting edges that are sharp and strong to cut through hair over an extended period of time. The continuous grinding process generally limits blade shapes to have straight edges with a substantially triangular or wedge shape profile (e.g., cross section). The cutting edge wedge-shaped configuration typically has an ultimate tip with a radius less than about 1000 angstroms.
The advantage of this prior method is that it is a proven, economical process for making blades in high volume at high speed. It would be particularly desirable if such a process could utilize lower cost materials for blade formation and also enable cutting edged profiles other than substantially triangular.
Blades with cutting edges made from a polymeric material are disclosed for disposable cutlery or disposable surgical scalpels (e.g., U.S. Pat. Nos. 6,044,566, 5,782,852). Razor blades made from polymeric material are disclosed in GB2310819A. The disadvantage of any of the prior art polymer blades is that the process of making such plastic blades is not cost-effective for mass production nor suitable to create a cutting edge with a tip radius of less than 1 μm as required for cutting hair.
Generally, the prior art utilizes melt flow processing techniques. The molten polymer of the prior art is injected into a cavity of a mold tool which is typically metal, but the polymer is generally too viscous (typically exceeding 100,000 centiPoise) to fully penetrate into the sub-micro-meter (e.g., less than 1 micrometer) dimensioned spaces required in a cavity to create razor blade edges. However, choosing a lower viscosity material or increasing the injection pressure, which may benefit penetration into sub-micro-meter dimensioned spaces, causes the polymeric material to penetrate between the mating surfaces of the two halves of the mould tool, known as “flashing,” and therefore the required cutting edge tip radius cannot be achieved. A decrease of viscosity of the polymeric material may also be obtained by heating the polymeric raw material above the glass transition temperature, often exceeding 200° C. Furthermore, after filling the cavity, the fluid polymeric material needs to be cooled to achieve a solid state, which causes shrinkage of the blade shape and rounding of the edge and therefore the required cutting edge tip radius cannot be achieved.
Therefore, a need exists for better processes for cutting edge structures made of polymer and more cost-effective methods of making cutting edge structures for shaving razors having required tip radius, less variability in edge quality and sharpness to provide a comparable or improved shaving experience.
It is also desirable to find materials and processes that can form cutting edge structures having any shape, such as non-linear edges and/or provide an integrated assembly.
Recently additive manufacturing techniques, such as stereo lithography and 3-dimensional printing have become widely used to fabricate polymeric structures. In both cases, a 3-dimensional object is build up from small volume elements, so-called voxels, of material that are successively added to each other until the entire object is formed. However, the spatial resolution of these techniques is limited to the size of an individual pixel of tens of micro-meters, which is greater than the ultimate tip radius of a cutting edge.
High resolution additive manufacturing, such as 2-photon polymerization (2PP) described for instance in Photonics Spectra Vol. 40 (2006), Issue 10, Pages 72-80, is known and its potential to create sub-micron sized objects has been demonstrated for micro-mechanical actuators (e.g., U.S. Pat. No. 7,778,723B2), micro-fluidics devices, optical elements (e.g., U.S. Pat. No. 8,530,118B2), photonic crystals (e.g., US2013/0315530A1) and bio-medical applications such as micro-needles (e.g., US Patent Publication No. 2009/099537A1, CN103011058A) and tissue engineering scaffolds (e.g., US Patent Publication No. 2013/012612A1).
All of these structures make use of high resolution additive manufacturing on very small object length scales (e.g., 1 mm or less). One disadvantage of this process is that a certain time is required to create each individual voxel and hence the overall size of the complete object determines the time required for its fabrication. Therefore, a need exists to fabricate larger objects, such as razor blades, using high resolution additive manufacturing on faster or more reasonable time scales.
Another disadvantage of high resolution additive manufacturing is that internal stresses occur due to the slight shrinkage of the polymeric material during curing. When objects with overall dimensions exceeding about 1 mm are fabricated by high resolution additive manufacturing, these internal stresses scale with size, and objects which are greater than 1 mm in size become unstable. Hence, there is a need to fabricate objects such as razor blades using high resolution additive manufacturing without internal stresses.